Primary Cellular Arm Spacing

Speaking about sub-grain structure and cells, another parameter to consider is the dimension of the primary cellular arm Spacing (PCAS). Increasing the energy density, the primary cellular arm spacing is expected to grow [20].

In the Table 15 and 16 below follow the results obtained measuring the PCAS of the samples selected for the microstructural analysis.

Table 15: PCAS measured with triangle method

Sample 20-75-00 05-75-30 04-75-45

Segments [➭m] Local PCAS [➭m] Segments [➭m] Local PCAS [➭m] Segments [➭m] Local PCAS [➭m]

2,992 3,401 3,494 4,154 4,196 7,161

3,778 4,462 8,854

3,432 4,506 8,434

2,996 2,484 3,314 3,809 9,024 7,994

2,391 3,838 5,017

2,065 4,276 9,942

2,782 3,248 3,705 3,735 6,684 7,072

3,835 3,905 8,764

3,126 3,595 5,769

2,757 3,263 3,719 3,64 5,71 8,047

3,66 3,411 9,813

3,372 3,79 8,617

Average PCAS [➭m] 3,099 3,835 7,569

Table 16: PCAS measured with triangle method

Sample 30-100-00 10-100-30 07-100-45

Segments [➭m] Local PCAS [➭m] Segments [➭m] Local PCAS [➭m] Segments [➭m] Local PCAS [➭m]

1,573 2,103 3,969 5,104 5,94 5,822

2,146 5,069 4,303

2,59 6,273 7,223

4,07 3,547 4,551 4,894 4,419 4,924

2,757 4,687 5,121

3,813 5,444 5,232

2,253 3,402 5,16 4,799 5,436 5,231

3,962 5,138 6,288

3,99 4,099 3,969

3,586 3,21 5,965 5,571 3,878 4,183

3,45 4,794 3,969

2,595 5,954 4,702

Average PCAS [➭m] 3,065 5,092 5,040

The results are contrasting, on one side there seems to be a slight increase in cell size between the two energy densities considered (in this case 07-100-45 would be an exception), on the other the “local” PCAS results are very different according to the area of the sample where the measurement is taken. Certainly, the primary cellular arm spacing increases as the oscillation amplitude of the head increase, lowering the cooling rate of the process.

More definitive results could be obtained repeating the measurements on more locations across the sample. Probably the number of measurements used in this study is not enough to obtain a full picture of the problem.

Figure 43: Primary Cellular Arm Spacing vs Oscillation amplitude

In Figure 43 the primary cellular arm spacing values related to the oscillating amplitude are reported. As noticed before, the cellular morphology seems to get coarser increasing the beam oscillation angle, but it is not clear from this data the relationship between energy density and PCAS size.

6 Conclusions

The main focus of this thesis is the optimization of a Direct Energy Deposition additive process for AISI 316L stainless steel with an oscillating scan strategy.

Major findings are listed as follows:

1. Through SSTs, stable process windows for both the Superficial Energy Den-sities (EDS) investigated in this study were found. Increasing the energy density was possible to print successfully at higher beam oscillation ampli-tudes.

2. After selecting the best set of parameters from the previous phase, it was pos-sible to successfully complete the deposition of twelve 20x20x20mm³ cubes with oscillating scan strategy amplitudes ranging from zero to sixty degrees.

3. The depositions realized with an oscillating scan strategy were proven to be faster than the traditional ones. Among our depositions, thanks to the process parameters chosen, this was more evident in the 75 J✴mm² case.

4. Most of the samples, whilst presenting a very rough surface finish, were substantially straight. The majority of geometrical inaccuracies was found for higher beam oscillation angles.

5. The analysis of the porosity of the samples was very successful. The porosity of the samples deposited with an oscillating scan strategy is comparable with the one of the samples printed traditionally. In agreement with literature, higher EDS seem to lead to smaller porosity values.

6. From microscope observation, it is evident that the size of the melt pools increases with the beam oscillation amplitude and the energy density.

7. Regarding grains, the samples mostly exhibit a heterogeneous and coarse grain structure. Columnar grains developed in the growth direction of the sample were present only with high oscillation angles. Both the morphologies are comparable with what was found in literature.

8. At sub-grain level we found a fine cellular microstructure with cells having different orientations. Primary cellular arm spacing grows increasing the amplitude of the laser beam oscillation, while there is not enough data to establish a clear relationship between PCAS and Superficial Energy Density.

In conclusion, after an accurate selection of parameters, it is possible to successfully print dense and accurate (for DED standards) simple shapes with an oscillating scan strategy. This allows for shorter deposition times and can provide an option to obtain different microstructures and spot sizes with the same machine and printing head.

With further studies it’s totally feasible to start the deposition of more complex shapes using an oscillating scan strategy, implementing IT tools for the complete industrialization of the process.

7 Bibliography References

[1] Yi Zhang et al. “Additive manufacturing processes and equipment”. In: Addi-tive Manufacturing: Materials, Processes, Quantifications and Applications.

2018, pp. 39–51. isbn: 9780128121559. doi: 10.1016/B978-0-12-812155-9.00002-5.

[2] Yeon-Gil Zhang, Jing; Jung. “Overview of additive manufacturing process”.

In: Additive Manufacturing - Materials, Processes, Quantifications and Ap-plications. isbn: 978-0-12-812155-9.

[3] ISO. “Additive manufacturing — General principles — Fundamentals and vocabulary (ISO/ASTM 52900:2021(en) )”. In: International Organization for Standardization (2021). url: https://www.iso.org/obp/ui/#iso:

std:iso-astm:52900:ed-2:v1:en.

[4] Daniel B¨ockin and Anne Marie Tillman. “Environmental assessment of addi-tive manufacturing in the automoaddi-tive industry”. In: Journal of Cleaner Pro-duction 226 (2019), pp. 977–987. issn: 09596526. doi: 10.1016/j.jclepro.

2019.04.086.

[5] Prima Additive. About Us. url: https://www.primaadditive.com/en/

company/about-us.

[6] Prima Additive. LASERDYNE➤ 811. url: https://www.primapowerlaserdyne.

com/laserdyne-811/.

[7] Prima Additive. LASERDYNE➤ 811 Technical Specifications.

[8] Alessandro Salmi. “Additive Manufacturing Processes for Metals”. In: Ad-ditive and Design for AdAd-ditive. Turin, 2021.

[9] Michael Zenou and Lucy Grainger. Additive manufacturing of metallic ma-terials. 2018, pp. 53–103. isbn: 9780128121559. doi: 10.1016/B978-0-12-812155-9.00003-7.

[10] Prima Additive. Print Genius 250. url: https : / / www . primaadditive . com/it/tecnologie/powder-bed-fusion/print-genius-250.

[11] Prima Additive. Laser Next 2141. url: https://www.primaadditive.com/

en/technologies/direct-energy-deposition/laser-next-2141.

[12] D. R. Feenstra et al. “Critical review of the state of the art in multi-material fabrication via directed energy deposition”. In: Current Opinion in Solid State and Materials Science 25.4 (2021), p. 100924. issn: 13590286. doi:

10.1016/j.cossms.2021.100924. url: https://doi.org/10.1016/j.

cossms.2021.100924.

[13] Sergey V. Kuryntsev and A. Kh Gilmutdinov. “The effect of laser beam wobbling mode in welding process for structural steels”. In: International Journal of Advanced Manufacturing Technology 81.9-12 (2015), pp. 1683–

1691. issn: 14333015. doi: 10.1007/s00170-015-7312-y.

[14] Nicola Ripoli. “Ottimizzazione del processo di Directed Energy Deposition con strategia di scansione oscillante di un acciaio AISI 316L”. PhD thesis.

Politecnico di Torino, 2021.

[15] Florian Fetzer et al. “Reduction of pores by means of laser beam oscillation during remote welding of AlMgSi”. In: Optics and Lasers in Engineering 108.May (2018), pp. 68–77. issn: 01438166. doi: 10.1016/j.optlaseng.

2018.04.012. url: https://doi.org/10.1016/j.optlaseng.2018.04.

012.

[16] Artem Aleksandrovich Voropaev et al. “Influence of Laser Beam Wobbling Parameters on Microstructure and Properties of 316L Stainless Steel Multi Passed Repaired Parts”. In: Materials 15.3 (2022). issn: 19961944. doi: 10.

3390/ma15030722.

[17] I. Hernando et al. “Laser beam welding analytical model when using wobble strategy”. In: International Journal of Heat and Mass Transfer 149 (2020), p. 119248. issn: 00179310. doi: 10.1016/j.ijheatmasstransfer.2019.

119248. url: https://doi.org/10.1016/j.ijheatmasstransfer.2019.

119248.

[18] Junzhao Li et al. “Analysis and improvement of laser wire filling welding process stability with beam wobble”. In: Optics and Laser Technology 134.2

(2021), p. 106594. issn: 00303992. doi: 10 . 1016 / j . optlastec . 2020 . 106594. url: https://doi.org/10.1016/j.optlastec.2020.106594.

[19] Abdollah Saboori et al. “Microstructure and mechanical properties of AISI 316L produced by directed energy deposition-based additive manufacturing:

A review”. In: Applied Sciences (Switzerland) 10.9 (2020). issn: 20763417.

doi: 10.3390/app10093310.

[20] Mingming Ma, Zemin Wang, and Xiaoyan Zeng. “A comparison on metal-lurgical behaviors of 316L stainless steel by selective laser melting and laser cladding deposition”. In: Materials Science and Engineering A 685.October 2016 (2017), pp. 265–273. issn: 09215093. doi: 10.1016/j.msea.2016.12.

112. url: http://dx.doi.org/10.1016/j.msea.2016.12.112.

[21] William D. Callister and David G. Rethwisch. Materials Science and Engi-neering: An Introduction. 2008. doi: 10.1063/1.2982126.

[22] Abdollah Saboori et al. “An investigation on the effect of deposition pattern on the microstructure, mechanical properties and residual stress of 316L pro-duced by Directed Energy Deposition”. In: Materials Science and Engineer-ing A 780.January (2020), p. 139179. issn: 09215093. doi: 10.1016/j.msea.

2020.139179. url: https://doi.org/10.1016/j.msea.2020.139179.

[23] ESMAT. A.6.3. Stainless Steel AISI 316L. url: http://esmat.esa.int/

Services/Preferred_Lists/Materials_Lists/a63.htm.

[24] ASTM International. Astm a240. 2004. url: https : / / www . astm . org / a0240_a0240m-22a.html.

[25] Alberta Aversa, Giulio Marchese, and Emilio Bassini. “Directed energy de-position of aisi 316l stainless steel powder: Effect of process parameters”. In:

Metals 11.6 (2021). issn: 20754701. doi: 10.3390/met11060932.

[26] Federico Mazzucato et al. “Influence of process parameters and deposition strategy on laser metal deposition of 316L powder”. In: Metals 9.11 (2019), pp. 11–14. issn: 20754701. doi: 10.3390/met9111160.

[27] K. Saeidi et al. “Hardened austenite steel with columnar sub-grain struc-ture formed by laser melting”. In: Materials Science and Engineering A 625 (2015), pp. 221–229. issn: 09215093. doi: 10.1016/j.msea.2014.12.018.

url: http://dx.doi.org/10.1016/j.msea.2014.12.018.